The present invention relates to the production of contact lenses. It relates more particularly to the production of contact lenses by the polymerization of a polymerizable composition in a mold which imparts to the polymerized composition the shape and size dimensions desired of a contact lens. Specifically, the present invention relates to the stage in such a production scheme wherein the polymerization of the polymerizable composition actually occurs.
Those familiar with the field of producing polymerized articles from polymerizable compositions are aware in general that compositions comprising one or more olefinically unsaturated monomers together with a small but effective amount of a polymerization initiator which is responsive to ultraviolet radiation of a given intensity and/or wavelength can be initiated by exposure of the composition to the indicated wavelength and intensity of ultraviolet radiation. However, it is also generally recognized that control of the initiation and propagation of polymerization is difficult. Accordingly, in many conventional applications of ultraviolet-initiated polymerization, the polymerization has been sought to be controlled by variations of the identity and/or amount of ultraviolet initiator, the presence and/or amount of polymerization inhibitor, and the characteristics of the single burst of ultraviolet radiation used to initiate the polymerization itself.
Because of the relative difficulty of controlling the polymerization characteristics and the properties of the resultant polymerized article, it has generally been considered that ultraviolet-induced polymerization is not completely satisfactory as a means for producing polymerized articles which must pass exacting requirements as to the dimensions and the physical properties such as structural integrity, surface smoothness, freedom from discernable irregularities in the internal structure and surface finish, clarity, transparency, and the like. Contact lenses are noteworthy examples of such articles which are subject indeed to numerous exacting criteria, by virtue of both the physical reality that contact lenses with even minor defects are considered unwearable, and of the regulations governing such articles prescribed by the U.S. Food and Drug Administration, among other national and international authorities.
There is therefore a need for a method and means for producing articles such as contact lenses via the ultraviolet-induced polymerization of compositions comprising olefinically unsaturated monomers.